Diffractometry relates to the irradiation of a material or object using a source of electromagnetic radiation, of X or gamma type, followed by the analysis of radiation from elastic scattering at a small angle. The expression “radiation from elastic scattering at a small angle” designates the radiation coherently scattered by the material or object at an angle less than 15°, or less even than 10°, relative to the direction of the radiation that is incident on the material or object. As a matter of fact, beyond 10°, elastic scattering, or Rayleigh scattering, becomes progressively negligible.
It is known to use diffractometry to detect certain crystalline substances such as most explosives or numerous other dangerous or illegal structures.
The invention thus finds an application in the field of security, more particularly in the field of the detection of explosive materials in an item of baggage.
It is also useful in the medical field, for example for locating a tumor in a breast. More particularly, a publication by UCL (Pani, S. et al. “Characterization of breast tissue using energy-dispersive X-ray diffraction computed tomography”. Applied Radiation and Isotopes 68, No. 10 (2010): 1980-1987) has been able to show the possibility of differentiating (adipose tissues, fibrous tissues, benign tumors, fibroadenomas, carcinomas, etc.) objects of biological tissues from breast biopsies thanks to the diffraction measurement of these tissues.
The analysis of the radiation scattered at a small angle (it is to be noted that that the term “diffracted” is generally used for a crystalline material, whereas the term “scattered” is generally used for an amorphous material, but these two terms here are used interchangeably, and likewise for the terms scattering and diffraction) by a material is a method of physico-chemical analysis which provides information on the structure of the material thereby enabling better characterization of materials.
It is known that the analysis of the spectrum of the radiation scattered at a small angle, or scattering spectrum, makes it possible to establish a signature for the material examined.
For crystalline materials for example, when the wavelength of the irradiating X-rays is of the same order of magnitude as the interplanar spacing (a few angstroms), the scattered rays generate constructive or destructive interferences according to their energy and their scattering angle. The conditions for which the interferences are constructive are determined by Bragg's law. For a crystalline material, this law links the interplanar spacing, the scattered radiation energy and the scattering angle, according to the following equation:
with:
      E    hkl    =      n    ⁢          hc              2        ⁢                  d          hkl                ⁢                  sin          ⁡                      (                          θ              /              2                        )                                  dhkl: interplanar spacing between the crystallographic planes of the irradiated crystal;    {hkl}: Miller indices    θ: scattering angle, that is to say the angle formed between the scattered ray analyzed and the beam that is incident on the irradiated crystal    h: Planck's constant    c: the speed of light    n: the order of the interference.
It is possible to identify the Bragg peaks by the momentum transfer defined by the following equation:
  x  =                    sin        ⁡                  (                      θ            /            2                    )                    λ        =          n              2        ⁢                  d          hkl                    
The interest in expressing the scattering profiles (measured intensity) according to x is due to the fact that an intensity peak may be measured for different pairs (λ,θ) but for a single value of x (n fixed).
In the case of non-crystalline materials, the spectrum for scattering at a small angle is also representative of the material examined.
By way of examples, appended FIG. 1 shows the Bragg peaks of two crystals, TNT (trinitrotoluene) and salt (NaCl).
In the manner of the interferences determined by Bragg's law for a crystalline material, interference phenomena may also occur between the atoms and/or molecules of an amorphous material such as a liquid, this time involving a known distribution of distances (molecular interference function, denoted MIF). As a matter of fact, many amorphous materials have regular arrangements over nanometric distances (the expression short-range order used). This type of order is determined by strong chemical bonds for the covalent and ionic bonds. This short-range order causes intramolecular and intermolecular interferences. Appended FIG. 2 illustrates examples of molecular interference functions, i.e. the normalized MIF for water (H2O), the normalized MIF for oxygenated water (H2O2), the normalized MIF for acetone (C2H6CO), and the normalized MIF of a material known under the tradename Plexiglas® ((C5O2H8)n).
The most common diffractometers are referred to as ADXRD (acronym for “Angular Dispersive X-ray Diffraction”). The energy is fixed by the use of monochrome radiation and the number of photons diffracted is measured according to the angle. Although these devices are very accurate, they require the use of a powerful monochrome source and cannot be used for imaging on account of their bulk.
Developed more recently, the EDXRD technique (EDXRD being the acronym for “Energy Dispersive X-Ray Diffraction”) enables these difficulties to be alleviated. This time they consist in working at a fixed angle and using a set of collimators to illuminate the object with a polychromatic beam to measure the diffracted photons with an energy resolved spectrometric detector. The diffraction peaks then appear at certain energies in the measured spectrum.
The EDXRD technique, and more generally any technique of analysis by spectrometry, requires the employment of a spectrometric detector that is sufficiently energy resolving to enable the separation and the identification of the different characteristic peaks of the material constituting the object to analyze. The known detectors having the best energy resolution are of the Germanium type. However, this type of detector must be cooled to very low temperatures, by complex and/or costly methods (thermoelectric cooling or cooling by a tank of liquid nitrogen). Also, the analysis devices employing such a detector are very bulky.
The recent emergence of spectrometric detectors capable of being used at ambient temperature, such as detector types implementing CdTe, CdZnTe, or scintillator materials, provides an attractive alternative to the Germanium detectors. To be precise, these detectors are compact, not cooled and less costly. However, their performance in terms of energy resolution is still less than that obtained with the Germanium detectors, even though quite good.
To know whether a given crystalline or amorphous substance is contained in an object, it is thus known to:                irradiate the object using an incident beam, emitted by a source of ionizing radiation, preferably collimated by a primary collimator,        detect the diffracted radiation using a detection device comprising                    a detector, here termed spectrometric detector, configured to establish an energy spectrum (or spectrum of energy) of the radiation scattered at a given scattering angle, that is to say a detector comprising                        a detector material capable of interacting with radiation scattered by the object and which, on the near side to the object, presents what is referred to as a detection plane,        a spectrometry measurement means, configured to measure an energy released by each interaction of a photon with the detector material and to establish at least one energy spectrum.                    a collimator, referred to as detection collimator, associated with the detector, the detector and the detection collimator being arranged so as to have a detection axis D forming a scattering angle θ with the central axis Z of the incident beam,                        analyzing the measured spectrum or spectra by comparison with the energy spectrum of the substance searched for.        
In general terms, an energy spectrum illustrates the energy distribution of radiation in the form of a histogram representing the number of photon interactions in the object (along the y-axis) according to the released energy (along the x-axis). Often, the energy axis is discretized into channels of width 2 δE, a channel Ci, centered on the energy Ei corresponding to the energies comprised between Ei−δE and Ei+δE.
Patent application WO2013098520 describes a device for analyzing a material based on the processing and the analysis of a scattering spectrum.
The invention is an improvement of such a method of analyzing a scattered spectrum, making it possible to have more precise information as to the signature of the object examined.
For this, the invention provides a method for analyzing an object using a detection system comprising a spectrometric detector, the method comprising the following steps:                irradiating the object with incident photon radiation, using a source of electromagnetic radiation;        acquiring an energy spectrum, referred to as measured scattering spectrum, scattered by the object at a given angle θ comprised between 1° and 15°, using the spectrometric detector, placed for scattering,        
The method according to the invention is characterized in that it further comprises the following steps:                acquiring a spectrum of energy transmitted by the object, referred to as measured transmission spectrum, using a spectrometric detector placed for transmission; the spectrometric detector placed for transmission may be that used to acquire the measured scattering spectrum, and which is moved between its two acquisition operations, either manually, or automatically using configured mechanical and information technology means; as a variant, a detection system is used comprising an additional spectrometric detector, which is then placed permanently for transmission and is dedicated to the acquisitions of measured transmission spectra;        reconstructing a function, referred to as signature, representative of the object, based both on the measured scattering spectrum and the measured transmission spectrum;        comparing the signature so reconstructed with signatures of calibration materials stored in a database, for the purposes of identifying a material constituting the object.        
The invention is thus based on the combined use of at least one scattering spectrum and a transmission spectrum. This combination makes it possible to obtain a more precise signature (Bragg peaks or Molecular Interference Function) of a material constituting the object.
Advantageously and according to the invention, the step of reconstructing a signature of the object implements a method based on an inverse problem type approach.
The implementation of a method based on an inverse problem type approach makes it possible to recover the signature specific to the material independently of the instrumentation factors so as to improve the identification of the material.
Advantageously, the step of reconstructing a signature of the object comprises an operation of constructing an overall response matrix A of the detection system, each term A(j,k) of the overall response matrix A corresponding to a probability of detecting, with the detector placed for scattering, a photon of energy j when the object produces a momentum transfer equal to k.
Generally the overall response matrix A establishes a relationship between energy detected by the detector placed for scattering and a parameter that is characteristic of elastic scattering of the material constituting the object analyzed, in particular the momentum transfer.
The overall response matrix A is produced taking into account:                the spectrum measured by the spectrometric detector placed for transmission, referred to as transmission spectrum, this spectrum representing the radiation emitted by the source into the object examined,        an angular response function establishing a relationship, for a given scattering angle, between the energy of a scattered radiation and a parameter that is characteristic of the elastic scattering of the radiation the object, in particular a momentum transfer.        
According to an embodiment, the response matrix A is produced by estimating the spectrum of the radiation from the source after attenuation by the object, such a spectrum being estimated from the transmission spectrum, for example using a calibration matrix of the detector placed for transmission;
According to an embodiment, the response matrix A is produced taking into account a calibration matrix of the detector placed for scattering.
In a preferred embodiment, the spectrometric detector placed for scattering is arranged so as to present a detection axis forming, with a central axis of the incident radiation, a scattering angle θ comprised between 1° and 10° or even between 1° and 5°.
In the most effective version of the invention providing the most precise signatures, the step of reconstructing a signature of the object comprises a step of estimating an incident spectrum attenuated by the object using the spectrum measured for transmission. Given the small scattering angle, the inventors have found that the attenuation by the object, as appearing through the transmission spectrum, could be advantageously taken into account in the processing of the scattering spectrum.
In a preferred embodiment, the method of analysis according to the invention comprises a first prior step of calibrating a response matrix of the spectrometric detector placed for scattering, a second prior step of calibrating a response matrix of the spectrometric detector placed for transmission, and a third prior step of calibrating an angular response matrix of the detection system, it being possible for these first, second and third prior calibrating steps to be carried out in one order or in another, before any irradiation of an object to analyze.
The prior steps of calibrating the response matrices of the spectrometric detector placed for scattering and of the spectrometric detector placed for transmission are not necessary but are advantageous, since they take into account the degradation of the spectra due to the response of the detector. However, these steps are optional, in particular for detectors that are sufficiently energy resolving and when the response of a detector is judged to be satisfactory.
In a preferred embodiment, the first and second prior steps of calibrating are carried out by simulation using a simulation software application of Monte-Carlo type.
The response of a detector, whether it be the detector used for transmission or the detector used for scattering, may be refined by exposing the detector to a source of radiation that is monochromatic or comprises a small number of emission rays, for example 241Am or 57Co.
According to an additional possible feature, the measured transmission spectrum and the measured scattering spectrum are acquired in a same operation of irradiating the object, provided a detection system comprising two sensors is available.
As a variant and preferably, the analyzing method comprises two parts executed in two stages: a first part during which the measured transmission spectrum is acquired and further to which the meeting of a suspicion criterion is verified, and a second part during which the measured diffraction spectrum is acquired and the signature is reconstructed. In this variant, it is possible to use a detection system with two detectors or a detection system with a single detector (which is moved from one position to the other).
More specifically, the first part of such a method in two stages comprises the following steps:                irradiating the object and acquiring a measured transmission spectrum,        determining a first characteristic of the object from the measured transmission spectrum; this first characteristic of the object may be a form, a dimension, an attenuation contrast, an effective atomic number Zeff of a material constituting the object;        Verifying the meeting of at least one suspicion criterion relating to the first characteristic of the object and expressing the fact that the object contains a potentially suspicious material for an application considered; thus for example, in the case of an application relating to the field of security such as the search for explosives in baggage, the suspicion criterion expresses the fact that the object contains a material of which the first characteristic is similar to that of an explosive material; in the case of a medical application such as the search for a tumor in a breast, the suspicion criterion expresses the fact that the object contains a material of which the first characteristic is close to that of a cancerous tissue. The suspicion criterion preferably defines a range of values of the first characteristic within which it is considered that the object is potentially suspicious, having regard to the application considered, and outside of which the object is considered as not being suspicious. The step of verifying the meeting of the suspicion criterion then comprises a comparison between the first characteristic determined for the object and the predetermined range of values having regard for the application considered.        
The second part of the method in two stages is executed only when the suspicion criterion is met and comprises:                irradiating the object and acquiring a measured scattering spectrum,        reconstructing the signature of the object based on the measured scattering spectrum (acquired during this second part) and on the measured transmission spectrum (acquired during the first part), as explained earlier,        comparing the first characteristic of the object and its signature with characteristics and signatures of calibration materials stored in a database, for the purposes of identifying a material constituting the object.        
Conversely, when the suspicion criterion is not met, the analysis method for the object is made to terminate at the end of the first part: the object in course of analysis is considered as inoffensive and is removed from the object receiving zone of the detection system. Where required, it is replaced by a new object to analyze.
The objective of the first part of such a method in two stages is not to identify the material constituting the object but to eliminate the objects that are clearly inoffensive (clear absence of explosive or clear absence of tumor or other unhealthy tissue, according to the application concerned) in order to save time, the second part of the method (longer since it requires the acquisition of a scattering spectrum) only being carried out on the potentially suspicious objects. The measured transmission spectrum is used twice in this version of the method according to the invention: a first time in the first part of the method for determining a characteristic of the object (form, dimension, effective atomic number, etc.) on which the suspicion criterion is based, and a second time in the second part of the method for reconstructing the signature.